1. Field of the Invention
The present invention relates to a method of producing the rubber thread for use in a golf ball and a rubber thread for a golf ball.
2. Description of the Related Art
The golf ball is classified into a wound golf ball whose core consists of a center piece and rubber thread wound around the center piece and a solid golf ball whose core consists of solid rubber. The wound golf ball gives a good feeling to a golfer when the golfer hits it and can be controlled in its flight distance, its flight direction and the like, whereas the solid golf ball can make a long flight and is durable. The wound golf ball has been used for a long time. In some days, the wound golf ball was used as the high-class golf ball. However, the solid golf ball developed later can be produced easily and at a low cost. Thus, in recent years, the solid golf ball is put on the market more than the wound golf ball. In such a situation, many professional golfers and high-class amateur golfers still like the wound golf ball because they can control the wound golf ball better than the solid golf ball in its flight distance, its flight direction and the like.
As one of the methods of producing the rubber thread for the wound golf ball, the so-called latex blend method has been used. In the latex blend method, rubber latex is mixed with a cross-linking agent and an additive to form a latex composition. Then, a conveyor belt having a liquid coagulating agent which has adhered to its surface is passed through the latex composition. The latex composition which has adhered to the conveyor belt is solidified to form a rubber sheet. The rubber sheet is cut to predetermined widths after the rubber sheet is cross-linked to produce the rubber thread.
The rubber thread obtained by the latex blend method has a uniform thickness. Thus, in winding the rubber thread while it is being stretched at 600% to 1000%, it is not broken easily. Because the rubber thread obtained by the latex blend method has a low modulus, it can be stretched sufficiently at a comparatively low tensile force. Therefore, when the rubber thread obtained by the latex blend method is wound as the core of the golf ball, it has a high density, which improves the repulsive performance of the golf ball.
However, the rubber latex which is used in the latex blend method is liquid. Thus, the latex blend method has a low work efficiency. Further, because the rubber latex is expensive, the material cost of the golf ball becomes expensive.
As the another method of producing the rubber thread, a dry blend method is known, as disclosed in Japanese Patent Application Laid-Open No. 56-34736. In the dry blend method, components such as solid rubber, a cross-linking agent, and an additive are kneaded with a kneader such as a Banbury mixer to form a rubber composition. The rubber composition is processed into a sheet material by a calender. The sheet material is cut to form the rubber thread. In the dry blend method, the kneader can mix the components easily with one another, and the producing process can be simplified. Thus, the dry blend method has a high operability.
However, the rubber thread obtained in the dry blend method has a non-uniform thickness and is liable to be broken when it is wound as the core of the golf ball while it is being stretched. To prevent the breakage of the rubber thread, means for making the average thickness thereof large is conceivable. However, the thick rubber thread stretches insufficiently, and thus the density of the rubber thread forming the core is low. To prevent the breakage of the rubber thread, means for using a large amount of an assistant cross-linking agent is also conceivable. However, in this case, the modulus of the rubber thread becomes high, and thus the density of the rubber thread forming the core is low. The golf ball having the core whose rubber thread has a low density has an insufficient hardness and thus a low coefficient of repulsion.
The present invention has been made in view of the above-described problems. Thus, it is an object of the present invention to provide a rubber thread, for a golf ball, which can be produced with a high workability, stretches easily, and is not broken easily when it is stretching.
To achieve the object, a method, according to the present invention, of producing a rubber thread for a golf ball comprises (a) an extruding process of extruding a rubber composition in the shape of a sheet by using an extruder to obtain a sheet material; (b) a rolling process of rolling the sheet material to make the sheet material thin to obtain a thin sheet material; (c) a cross-linking process of cross-linking the thin sheet material to obtain a cross-linked sheet material; and (d) a cutting process of cutting the cross-linked sheet material to obtain a rubber thread.
According to this method of producing, initially, the rubber composition is extruded in the shape of a sheet, and then the sheet material is thinned in the rolling process. Unlike the conventional dry blend method, the method of producing of the present invention allows the thickness of the thin sheet material, namely, the thickness of the rubber thread to be uniform. Accordingly, the rubber thread is not easily broken when it is wound as the core of a golf ball. Further, because the method of producing of the present invention allows the thickness of the rubber thread to be small, the rubber thread forming the core has a high density.
The sheet material obtained in the extruding process is rolled in the rolling process before a temperature of the sheet material does not drop. Thereby, it is unnecessary to heat the sheet material in the rolling process and thus possible to save energy. By performing the rolling process successively to the extruding process, it is unnecessary to secure a sheet material-storing place and thus space-saving.
Preferably, the thickness t1 of the sheet material obtained in the extruding process is set to the range of 1 mm to 8 mm both inclusive, and the thickness t2 of the thin sheet material obtained in the rolling process is set to the range of 0.3 mm to 0.6 mm both inclusive. Thereby, it is possible to make the thickness of the rubber thread small and uniform.
The ratio of the thickness t2 of the thin sheet material obtained in the rolling process to the thickness t1 of the sheet material obtained in the extruding process is set to a range of 0.03 to 0.5 both inclusive. Thereby, it is possible to make the thickness of the rubber thread uniform.